1. Field of the Invention
This invention relates to a light emitting device with a photonic crystal that exhibits at least an omnidirectional photonic band gap for a predetermined range of frequencies of electromagnetic radiation.
2. Description of the Related Art
U.S. Pat. No. 5,955,749 discloses a light emitting device including a dielectric structure that has at least a two-dimensional periodic variation in dielectric constant, and a radiation source that generates radiation within the dielectric structure. The dielectric structure exhibits a spectrum of electromagnetic modes including guided modes of frequencies below a predetermined cutoff frequency, and radiation modes of frequencies below and above the cutoff frequency. The guided modes correspond to those frequencies of electromagnetic radiation which are propagated in any direction in the plane of the dielectric structure. The radiation modes correspond to those frequencies which are propagated outside the dielectric structure. The two-dimensional periodic variation in dielectric constant introduces a photonic band gap between the guided modes, which permits coupling of the radiation resulting from the radiation source to the radiation modes rather than to the guided modes, thereby resulting in radiation extraction from the dielectric structure. The dielectric structure includes a periodic pattern of elements which introduce the band gap between the guided modes. The elements can be a periodic pattern of holes in a layer or a periodic pattern of posts spaced apart by inter spaces. The layer or each of the posts includes an n-type semiconductor layer, an active layer or a quantum well, and a p-type semiconductor layer.
Although the aforesaid conventional light emitting device can enhance the light emission efficiency, the introduced photonic band structure doesn't show a wide omnidirectional band gap. Especially, when the emitting light is unpolarized, the enhancement of the light extraction efficiency is limited.
U.S. patent application Publication No. US 2003/0141507, invented by Krames et at., discloses a light emitting diode (LED) using a photonic crystal structure. The light emitting diode includes a periodic structure, such as a lattice of holes, formed in the semiconductor layers of the LED. The parameters of the periodic structure are such that the energy of the photons, emitted by the LED, lies close to a band edge of the band structure of the periodic structure. The light emitting diode includes a first semiconductor layer coupled to a first electrode layer, an active layer overlying the first semiconductor layer, a second semiconductor layer overlying the active layer, and a second electrode layer overlying the second semiconductor layer. Although Krames et al. mentioned that the metal electrode layers, which respectively overlie the first and second semiconductor layers, have a strong influence on the emission efficiency of the light emitting diode, there is no discussion as to their influence on the band gap in the band structure of the periodic structure.
The entire disclosures of U.S. Pat. No. 5,955,749 and U.S. patent application Publication No. US 2003/0141507 are hereby incorporated herein by reference.